In microwave-assisted chemistry, microwaves are used to initiate, drive, or otherwise enhance chemical or physical reactions. Generally, the term “microwaves” refers to electromagnetic radiation having a frequency within a range of about 108 Hz to 1012 Hz. These frequencies correspond to wavelengths between about 300 cm to 0.3 mm Microwave-assisted chemistry is currently employed in a variety of chemical processes. Typical applications in the field of analytical chemistry include ashing, digestion and extraction methods. In the field of chemical synthesis, microwave radiation is typically employed for heating reaction materials, many chemical reactions proceeding advantageously at higher temperatures. In addition, when pressureriseable reaction vessels are used, many analytical or synthetical processes can be further enhanced by increasing the pressure in the vessel. Further, when, for example, digestion methods for analytical purposes are used, the generation or expansion of gases inside the vessel will necessarily increase the internal pressure. Thus, in order to ensure that no reaction products are lost for subsequent analysis, vessels must be used which are able to withstand high internal pressures in these cases.
Usually, most microwave-assisted reactions are performed in open or, preferably, in sealed vessels at temperatures rising up to 300° C. Typical pressures range from below atmospheric pressure, e.g. in solvent extraction processes, up to 100 bar, e.g. in digestion or synthesis processes.
Microwave-assisted chemistry is essentially based on the dielectric heating of substances capable of absorbing microwave radiation, which is subsequently converted into heat.
Many apparatuses and methods currently employed in microwave-assisted chemistry are based upon conventional domestic microwave ovens operating at a frequency of 2.45 GHz. As magnetrons operating at this frequency are produced in large quantities for domestic appliances, microwave apparatuses for microwave-assisted chemistry using such magnetrons can be manufactured at relatively low cost.
The applicator cavity of heating devices based on domestic microwave ovens is usually a multi-mode resonance cavity in which the spatial energy distribution is determined by an interference of standing waves of different longitudinal and transverse modes of the microwave field. Accordingly, an inhomogeneous field distribution results leading to so-called “hot spots” and “cold spots”, respectively. In order to ensure homogenous heating of the sample arranged within a multi-mode resonance cavity, the sample to be heated is usually arranged on a turntable which is rotated during the heating process in order to level the overall energy absorbed throughout the sample.
It is also known that depending on the sample loading in the cavity and on the dielectric characteristics (permittivity) of the sample, the balance between the electromagnetic modes within a multi-mode cavity and consequently the overall distribution of microwave energy within the cavity will be modulated. This will usually not pose a particular problem, because the rotation of the sample on the turntable during the heating process will still ensure a sufficient balancing of the overall energy absorbed by the sample. Consequently, except for a turntable, no special means are usually employed in a multi-mode cavity to compensate for field distribution changes caused by varying load characteristics.
Multi-mode applicator cavities based on household microwave ovens have a rather large sample volume and are consequently particularly suited to heat larger samples. For smaller sample volumes, other devices, namely so-called mono-mode or single-mode applicators are usually employed for microwave heating in chemical analytics or synthesis. A typical single-mode microwave heating device used is for instance described in U.S. Pat. No. 4,681,740. Such a typical single-mode microwave applicator used in chemical synthesis or analysis comprises a magnetron for generating microwave radiation, typically operating at a frequency of 2.45 GHz, having an antenna which extends into one end of an hollow rectangular waveguide. At microwave frequencies of 2.45 GHz, a so-called WR340 rectangular waveguide having internal dimensions of 86×43 mm, is commonly used, in which the TE10 mode of the microwave field can propagate. At the opposite end of the rectangular waveguide, a resonant applicator cavity is provided which is adapted to accommodate a sample vessel. Devices such as the microwave heating device of U.S. Pat. No. 4,681,740, are provided with a circular opening in the upper wall of the applicator cavity through which the sample vessel with the sample to be heated can be inserted into the cavity. A metallic cylindrical chimney extends above the opening. The diameter of the opening and the height of the chimney are selected such that no microwave radiation can escape from the waveguide through the opening into the chimney
As compared to multi-mode cavities, single-mode applicators tuned to resonance have the advantage that when operating at similar power levels, higher field intensities and a more even energy distribution throughout the sample can be achieved. In addition, as the ratio of sample volume to cavity volume is increased, the overall energy yield is also improved. However, in order to achieve these advantages, a good impedance matching of the impedance of the rectangular waveguide and the impedance of the applicator cavity has to be achieved in order to obtain an efficient energy transfer into the sample. However, as noted above, the impedance of the applicator cavity is influenced by the sample to be heated itself. Consequently, the heating of different samples having different permittivity or even the heating of a single sample which has a changing permittivity throughout the heating process, as well as using samples with different sample volumes, will effect the impedance characteristics of the cavity/sample-system and may therefore deteriorate the initial matching to the impedance of the waveguide.
In prior art, several solutions have been suggested to improve the absorption of the microwave radiation by the sample within an applicator cavity. For instance, in U.S. Pat. No. 5,382,414, a lifting device comprising a piston rod is described which allows to change the height of a plate on which the sample is arranged within the applicator cavity. U.S. Pat. No. 5,837,978 describes a multi-mode cavity, where resonance conditions can be improved by adapting the height of the applicator cavity to changing process conditions. WO 99/17588 A1 describes a device for controlling the feeding of microwave power through a waveguide into a microwave heating appliance by movably arranging a conductor member in the waveguide in order to affect the mode pattern of the microwave radiation transported through the waveguide. In US 2004/0069776, a waveguide comprising a rotatable deflector is described, which is controlled via a dummy load in order to maximize energy transmission into the sample cavity. Accordingly, prior art devices require sophisticated adjusting and control means to adapt microwave transmission to varying permittivity conditions in the applicator space. Consequently, the provision of such control systems leads to a considerable increase of the overall manufacturing costs of the microwave heating devices of prior art.